The overall objective of this study is to contribute to a better understanding of the basic mechanisms involved in mammalian fertilization. Using an improved technique we have developed, we will study (1) how extracellular and intracellular conditions of the spermatozoon effect capacitation and the acrosome reaction and (2) what kinds of chemical changes are happening in the sperm plasma membrane during capacitation and the acrosome reaction. The guinea pig will be used for these studies as a model animal. We will also study the process and mechanism of sperm-egg fusion using the hamster, guinea pig and mouse as model animals. We will elucidate the behavior and roles of (1) the equatorial segment of the sperm acrosome and (2) actin molecules (filaments) in both the spermatozoon and egg in sperm-egg fusion and post-fusion events. We will also study how extracellular and intracellular condition affect sperm-egg fusion.